Glucose is a simple sugar which is an important source of energy for the body and especially the brain. It is stored in the body in the form of glycogen. Normally, the glucose concentration in blood is maintained at approximately 5 mmol/l by hormones such as insulin and glucagon. Neurological and other symptoms such as hypoglycycemia can result if the blood-glucose concentration falls below this level. If, on the other hand, the blood-glucose level exceeds normal (e.g. above approximately 10 mmol/l) hyperglycemia, a symptom of diabetes mellitus, can develop. Therefore, it is extremely important that the concentration of glucose in the blood must be maintained at a proper level.
Unfortunately, some individuals are unable to maintain the proper level of glucose in their blood; perhaps due to disease or injury. In such cases, the blood-glucose concentration can generally be altered to bring it to a proper level; for example, through the use of insulin which decreases the amount of glucose in the blood. Conversely, glucose may be added to the blood by means of injection, an intravenous solution, or by eating/drinking certain foods/liquids. Of course, before the blood-glucose level concentration can be appropriately adjusted, the present or existing level must be accurately determined.
One viable technique for measuring glucose concentration involves applying a blood sample to a biosensor. A controlled voltage is then applied across the biosensor. The resulting electrochemical reaction causes a current to flow through the sample, the magnitude of which is related to the glucose concentration. This current is applied to an input of a current-to-voltage (I/V) converter circuit in, for example, a blood glucose meter. The I/V converter produces a voltage related to glucose concentration which may then be applied to an analog-to-digital (A/D) converter. The A/D converter, in turn, generates a precise (e.g. 10-16 bit) digital representation of the voltage supplied by the I/V converter. This digital representation may then be applied to a processor which interprets the digital representation by applying a previously determined calibration to quantitatively determine the blood-glucose level. This level may then be processed, stored to create a history, displayed, etc. Clearly, the accuracy of the resultant blood-glucose measurement is dependent to a large extent on the precision of the voltage generated by the I/V converter.
The IV converter may comprise an integrated operational amplifier which receives a reference voltage as an input and provides that reference voltage plus the amplifier's offset voltage (which can be made virtually negligible) to a first port or contact pad coupled to the blood sample. This first port is also coupled to a second port or contact pad, and the amplifier's output is coupled to a third port or contact pad. The current produced in the sample then flows through a high-precision external feedback resistor (i.e. external to the chip) which is coupled between the second and third ports or contact pads. Theoretically, the voltage drop across the external resistor would very accurately reflect the current produced in the blood sample. However, a problem arises because integrated circuits normally require electrostatic discharge (ESD) protection for all input ports or pads. A necessary portion of this protection comes in the form of on-chip parasitic resistances coupled to the ports or pads. These resistances can drift with temperature thus contributing error and variability to the current-to-voltage conversion process.
The problem is further complicated if a transfer gate having is own parasitic resistance is introduce into the circuitry of the I/V converter. Such a transfer gate may be required because there are a limited number of ports on the blood-glucose meter, and it may be necessary to use the above referred to first port or contact pad for other purposes which do not involve the I/V converter circuitry (e.g. communication with an on-chip processor, blood detection, etc.). The transfer gate acts as a switch which can be turned on and off to either electrically include or isolate the I/V converter. The transfer gate's parasitic resistance introduces additional error into the current-to-voltage conversion process.
Thus, a need exists for a high-precision current-to-voltage converter circuit for use in a blood-glucose meter, which circuit substantially reduces the effects of parasitic resistance on the resultant output voltage.